A kind of ultra-narrow dual-channel filter is proposed in principle and demonstrated experimentally. This filter is designed by means of two sampled fibre Bragg gratings (SFBGs), where one is periodic 0-π sampling and the other is symmetrical spatial sampling. The former can create two stopbands in the transmission spectra and the latter can produce two ultra-narrow passbands. Our filter has the 3-dB bandwidth of about 1 pm, whose value is two orders of magnitude less than the bandwidth of the traditional SFBG filters. The proposed filter has a merit that the channel spacing remains unchanged when tuning the filter.